Does bending the neck for hours to use smartpohone cause "graft popping" after hair transplant?

Bending your head does not cause grafts to pop out. If this is an issue you would be the first.  Follow up with your doctor for a recheck.

I ask my patients to be careful the night after their hair transplant, but most grafts are strong enough the day after procedure that bending cannot dislodge them.  Of course you still have to be careful with hair wash and touching the area.Here is a link that tells you who to care for your newly transplanted hair after hair transplant.  You need to educated yourself about all dos and don'ts after a hair transplant for the best result and a smoother experience.

Movement of the head and neck will not cause the grafts to come out. What will cause them to come out is picking, scratching or rubbing them inappropriately.